Amplifiers are a class of electrical circuits that accept an input signal vin and provide as an output a scaled version of the input signal vout=A·vin, where A is the gain, and vout and vin are provided as non-limiting example signals. Amplification is often realized using a species of transconductor, such as a transistor. In “single-ended” amplifiers, vin, is a single ground- or common-referenced signal, and vout is similarly ground- or common-referenced. In a differential amplifier, either the received vin or the produced vout is a signal encoded as the difference between two signal branches, also known as differential signal. In some differential amplifiers, a single-ended vin may be converted to a differential vout.
In many amplifiers, undesirable noise may be minimized by increasing the transconductance of the transconductors within the amplifier, which is commonly accomplished by increasing the bias current iB provided to the transconductors. A common practice in differential amplifiers is to provide a substantially symmetrical circuit, where two branches “mirror” one another as is the case in a differential pair. A single “tail” current iT may be drawn by a single pull-down current source from both branches. By setting iT=2iB substantially, it can be ensured that a relatively constant and predictable bias current iB is provided to each branch.